The present invention relates to a process for drying particles of hydrolyzed ethylene-vinyl acetate copolymer, more particularly to the process which provides the particles having excellent molding properties.
Hydrolyzed ethylene-vinyl acetate copolymer, having ethylene content of 20 to 50 % by mole, vinyl acetate content (including hydrolyzed portion thereof) of 80 to 50% by mole and degree of hydrolysis in vinyl acetate component of not less than 90% by mole, is suitably employed for the use of wrapping films or vessels for foods, because the oxygen permeability of the hydrolyzed copolymer is remarkably low as compared with other known snythetic resins. The hydrolyzed copolymer is also suitable for use as the so-called engineering plastics, such as machine parts, electric-device parts and vessels, because the antistatic property, resistances against oils and solvents, and mechanical strength are excellent.
The hydrolyzed ethylene-vinyl acetate copolymer has been prepared by hydrolyzing an ethylene-vinyl acetate copolymer in a medium in the presence of an alkaline catalyst, separating particles from the reaction mixture and drying the wet particles in air. If necessary, before drying, the wet particles are washed with water or an organic solvent to remove completely the catalyst and by-produced salts from the particles, and are subjected to an appropriate stabilization treatment. However, films or vessels melt-molded of the thus prepared copolymer have a number of fish eyes of 0.1 to 1 mm. in diameter. Also, in the case of continuing the molding for a long period of time, the melt viscosity of the hydrolyzed copolymer increases with the lapse of time. Therefore, it is very difficult to smoothly prepare a molded article having good appearance and properties by employing the hydrolyzed copolymer so prepared. Further, when machine parts and electric-device parts are prepared by means of injection molding, useless portions such as sprue and runner account for a good amount. The reuse of such useless portions has an effect on reducing the price of molded article, but the reuse of the hydrolyzed copolymer causes the lowering of, not only the molding property and the appearance, for example fish eye, coloration, of the obtained molded article but also sometimes the lowering of physical properties.
The fish eye is small swelling on film surface like eye of fish, and this decreases oxygen impermeability and impairs appearance of film. Great many factors of the generation of the fish eye can be considered in each step of polymerization, hydrolysis, drying and molding of the copolymer, and many attempts to solve the problem have been made. There has been, for example, proposed in Japanese Pat. publication No. 37665/1971 a process for drying particles of hydrolyzed ethylene-vinyl acetate copolymer in an atmosphere of inert gas such as nitrogen, carbon dioxide gas or mixture thereof, which contains not more than 5 % by volume of oxygen, at a temperature of not more than 95.degree. C. with agitation. This process has the advantage that the generation of fish eyes can be reduced in the molding of relatively short period of time. However, when molding is continued for a long period of time, fish eyes and the melt viscosity of the hydrolyzed copolymer increases with the lapse of time. Also, the reuse of such a hydrolyzed copolymer causes the viscosity increase and the coloration.